1. Field of the Invention
The present invention relates to air press assemblies of papermaking machines, and, more particularly, to rolls used in such air press assemblies.
2. Description of the Related Art
For many years attempts have been made to use external air pressure to force water out of a paper web. Rather than compress a sheet at a press nip to the point where hydraulic pressure drives water out, as is the case in normal wet pressing, it was reasoned that more water could be removed, and sheet bulk could be maintained, if air pressure could be applied to supplement roll nip generated hydraulic pressures. One such attempt involves providing a multi-roll or other structure forming an air press having a closed chamber, wherein air is circulated through the chamber to convect moisture out of the paper web.
One significant challenge that exists in designing an effective multi-roll air press assembly is in minimizing the leakage of air from the assembly adjacent each set of roll ends, each set of roll ends defining a respective chamber end. Typically, a respective seal assembly is held in tension against each chamber end. However, the opportunity for air leakage from the nip region between respective rolls exists, especially in the end portions of each roll not in contact with the paper web and at least one web transfer fabric being fed therethrough.
Another challenge that exists in designing an effective multi-roll air press assembly is in minimizing the cost, labor and down time associated with the replacement of a worn roll, especially large-diameter main rolls. Even if some type of a surface cover is used in such an instance, replacement involves removing the surface cover from the entire roll length and then placing on a new surface cover. The expense can be increased if the entire roll should instead require replacement.
What is needed in the art is a roll for use in an air press assembly of a papermaking machine that results in decreased air leakage from multi-roll air presses adjacent nip ends thereof, and has readily accessible and replaceable portions which tend to wear for a certain amount of time before any significant wear begins to occur on the primary part of the roll.
The present invention provides a roll for use in an air press of a papermaking machine which has a hard central section, in order to promote paper web processing, and soft, elastomeric ends, in order to reduce air leakage from the air press.
The invention comprises, in one form thereof, a roll for use in an air press assembly of a papermaking machine, the roll having a pair of ends associated therewith. The roll includes a pair of edge portions with each edge portion extending to one of the pair of ends. Each edge portion has an edge surface portion composed of a first material, the first material having a first hardness. The roll further includes a middle portion located between the pair of edge portions, the middle portion having a middle surface portion composed of a second material. The second material has a second hardness, the second material being harder than the first material.
The invention comprises, in another form thereof, a method of pressing a fiber web using an air press. The method includes the step of providing an air press for pressing a fiber web. The air press includes at least one cap roll including a first cap roll; and at least one main roll including a first main roll, the first cap roll and the first main roll forming a first nip therebetween, each main roll having a pair of ends associated therewith. Each main roll has a pair of edge portions, each edge portion extending to one of the pair of ends. Each edge portion has an edge surface portion composed of a first material, the first material having a first hardness. Each main roll also has a middle portion located between the pair of edge portions, the middle portion having a middle surface portion composed of a second material. The second material has a second hardness, the second material being harder than the first material. The method further includes the steps of forming a seal between the pair of edge portions of the first main roll and the first cap roll and feeding the fiber web through the first nip into the air press.
An advantage of the present invention is that the soft edge portions promote sealing of the respective nips of the plurality of rolls in an air press assembly, thereby reducing air leakage and increasing the effectiveness of the air press assembly.
Another advantage is the soft edge portions, which are replaceable, have a larger average diameter than and thus tend to wear out before the central hard portion, which is not as readily replaced as the soft edge portions. As such, with frequent enough replacement of the soft edge portions, the need for replacing the central hard portion is greatly reduced.
Yet another advantage is that the size of the soft edge portions can be chosen such that each soft edge portion is limited to the area where sealing is required and such that each does not interfere with the conveyance of a fiber web through the air press assembly.
An even farther advantage is that by placing the soft rubber on the edges of each of the larger-diameter main rolls instead of the smaller-diameter cap rolls, the soft rubber has an increased chance for a longer life as there is more chance for cooling, during a rotation, and less nip cycles per minute. Furthermore, more rubber is exposed to metal in each main roll, allowing greater heat transfer and thus better cooling.